Electric charge

Electric charge, also called electricity or energy, is a resource that is needed to operate various parts in the game. It is critical for unmanned spacecraft, which are generally uncontrollable without any electric charge. However, manned spacecraft usually also need some electricity, mainly for reaction wheels.

Engine alternators

Many rocket and jet engines have alternators installed which produce electricity. The exact output varies based on throttle level, and engines not running will not produce any electricity. The adjacent table shows the engines which provide electricity, their production at full throttle and fuel usage in vacuum. Jet engines only generate electricity when supplied with intake air.

Solar panels

A solar-powered satellite

Solar panels are a lightweight source of electrical energy. However, they need direct sunlight to work, so they won't produce electricity on the night side of a planet, in the shadow of a spacecraft or during eclipses of the sun, although the craft is illuminated while in an eclipse. Due to this unreliability, it is recommended to have some energy storage as a buffer when supplying a craft solely with solar panels.

All panels except for the OX-STAT Photovoltaic Panels and the OX-STAT-XL Photovoltaic Panels need to be extended using the right-click menu (Extend Panels option, and for the retraction choose the Retract Panels) or action groups (using the Toggle Panels command, the player can extend and retract several solar panels with a single keystroke) before they will generate energy. Unpacked solar panels are very fragile and will easily break off when colliding or experiencing atmospheric drag, so they must be retracted during liftoff, aerobraking, or atmospheric re-entry. Note that, as of version 1.0, the unshielded OX-4series solar panels cannot be retracted once deployed, so they are not suited for aerobraking missions.

The energy output of solar panels depends on their orientation to Kerbol. Except for the surface-mounted OX-STAT Photovoltaic Panels, all panels will automatically pivot around one axis to face the sun as much as possible. Reorienting a vessel to manually aim the panels at the sun and eliminate shadows cast on them will also improve power generation.

Generated power will also decrease with increasing distance from Kerbol, following the real-life inverse-square law:

Radioisotope thermoelectric generator

The PB-NUK Radioisotope Thermoelectric Generator is a constant and reliable source of energy which doesn't require any sunlight and is much more resistant to atmospheric drag than solar arrays. Unfortunately it has a very impractical shape and is significantly heavier than solar panels with a comparable output of electricity.

Fuel cells

The fuel cell, like the Fuel Cell and Fuel Cell Array, converts liquid fuel and oxidizer directly into electric charge, similarly to generators. As now the electricity production is the only purpose: they consume 0.0025 units of fuel for each electric charge unit (0,0025 /⚡), means they produce 400 units of electricity from a single unit of fuel.

Ion engines and rover wheels require particularly large amounts of electricity to operate.

If an unmanned craft has no electric charge available, it becomes entirely nonfunctional and no parts may be operated (notably, this includes motorized solar panels which might have allowed it to recover). However, it can still be saved if another craft docks to it and supplies it with electricity (using either a docking port or an Advanced Grabbing Unit) or if a kerbonaut on EVA manually extends the solar panels.